1. Field of the Invention
The present invention relates to a hydraulic controller of a power transmission device mounted on an automobile.
2. Description of the Related Art
Conventionally, a power transmission device mounted on an automobile transmits motor power of a motor, which is input, for example, through a torque converter, to drive wheels through a forward/backward change-over mechanism and a belt type continuously variable transmission. In such the power transmission device it is necessary to control an engagement capacity of friction engagement elements (a clutch for a forward move and a brake for a backward move (hereinafter simply referred to as “forward clutch” and “backward brake,” respectively) for configuring the forward/backward change-over mechanism in order to prevent an excessive torque from being input to a V belt; alleviate a transmission shock generated at the time of a start from a neutral, a forward/backward change-over, or the like; or to prevent a slip of the V belt at the time of a lock of the drive wheels. In the engagement capacity control of such the forward/backward change-over mechanism a linear solenoid valve (hydraulic pressure adjustment electromagnetic valve for the forward/backward change-over mechanism) is used to control a hydraulic pressure supplied to the mechanism (for example, see line 7 of a left column to line 5 the right column of page 3 and FIG. 1 in Japanese Patent Laid-Open Publication Hei 3-144160).
In addition, also in a lock-up clutch mechanism (direct coupling mechanism) of the torque converter, it is preferable that optimum engagement capacity control depending on a vehicle speed and the like is implemented, and even in a case of such the purpose the linear solenoid valve (hydraulic pressure adjustment valve) is used.
However, the linear solenoid valve is generally expensive, and there is a problem that cost extremely becomes high in the configuration with the solenoid valves as many as two as described above.
Then in the hydraulic controller of the power transmission device there is a problem that a layout thereof is restricted in order to place the solenoid valves and a space has to be largely ensured.
In addition, a high hydraulic pressure is always supplied to the friction engagement elements of the conventional forward/backward change-over mechanism and thus they are completely engaged. Therefore, because when a strong braking is implemented during an engine brake, a drastic load is given to the V belt of the transmission due to the complete engagement of the forward clutch, the V belt slips in some case.
Consequently, is strongly requested a hydraulic controller of a power transmission device, which controls the forward/backward change-over mechanism and the torque converter with one electromagnetic valve and prevents the slip of the V belt of the transmission.